"With Reid’s signature, a bipartisan majority of all senators now have signed the letter," said Sen. Bernard Sanders, I-Vt., who helped organize the effort, in a release touting Reid's move.

The moratorium would save postal jobs while giving Congress time to come up with a way to reform the postal service, which reported it lost $2 billion in the second quarter, and $740 million more than the same time last year.

Previous losses, and a lack of congressional action, have led the postal service to consolidate 141 mail-processing facilities since 2012 and more closures are expected. Sanders said a moratorium would save as many as 15,000 jobs.

Along with Sanders, Democratic Sens. Tammy Baldwin of Wisconsin and Jon Tester of Montana put the effort together and released a letter in August . Most of the 50 signatories to the letter were Democrats, but six were Republicans: Sens. Roy Blunt of Missouri, John Hoeven of North Dakota, John Thune of South Dakota, Susan Collins of Maine, James M. Inhofe of Oklahoma and Orrin G. Hatch of Utah.

After the letter was released, Sen. Thomas R. Carper, D-Del., who is chairman of the Homeland Security and Governmental Affairs Committee and has written a postal service overhaul bill, issued a release warning that in the absence of congressional reform, a moratorium would further drain the USPS's resources without giving it any additional authority to cut costs or increase revenue, and increasing the risk of a congressional bailout.

"If my colleagues want to address these concerns for the long-haul, I urge them to join me this September as we continue our efforts to fix the serious, but solvable, financial challenges facing the Postal Service," Carper said. "Our bill isn’t perfect but it is an important step in the right direction. I hope my colleagues will join our efforts to enhance this plan in order to save the Postal Service before it’s too late.”